In digital networks there is an on-going need to perform maintenance and diagnostic operations. One technique employed for this purpose is known as loopback. The transmit pair on a T1 card, for example, is connected directly back to the receive wire pair. If data on the T1 trunk is becoming corrupted, placing loopbacks at various points in the network can help isolate the fault.
In order to manage the network, some means must be provided for detecting when loopback occurs. One possibility is to use out-of-band signaling to detect the presence of loopbacks. However, services provided by some Carriers require that all twenty-four channels T1 trunk on a T1 carry 64Kbps data. Loopback detection using out-of-band signaling only allows twenty-three T1 channels to carry 64Kps data. Loopback detection using out-of-band signaling only allows twenty-three T1 channels to carry 64Kps data since one channel must be reserved for the out-of-bank signaling.
U.S. Pat. No. 4,059,729 discloses an arrangement for testing a communications channel by designating a subscriber channel with a control signal using a monitored data signal having the same data content as the multiplexed data stream.
An object of the invention is to remove this restriction.
A method of detecting loopbacks on trunks in digital networks, comprising the steps of detecting an out-of-service trunk; in response to the detection of an out-of-service trunk, initiating the continual placing of a predetermined bit pattern on at least one outgoing user channel on said out-of-service trunk at a network node while it remains out-of-service; continually monitoring at least one receive user channel on said trunk at said network node corresponding to said at least one outgoing channel to detect said bit pattern; and identifying the presence of a loopback when said bit pattern is persistently detected on said receive channel while said trunk is out of service.
The invention determines when a loopback has been placed on the trunk, which might be a T1, E1 or any other primary rate transmission medium that carries channelized information. This information is used by the equipment at the endpoints to determine if it is safe to switch back to the preferred trunk. If there are loopbacks, then the link status on the preferred path is not OK and the traffic remains on the alternate route.
Loopback detection occurs only when the link is out of service and none of the 24 channels are being used to carry user data.
The invention also provides an apparatus for detecting loopbacks on trunks in digital networks, comprising a detector for detecting an out-of-service trunk, means for initiating the continual placement of a predetermined bit pattern on an outgoing user channel in response to the detection of an out-of-service trunk at a network node by said detector while said trunk remains out of service, means for continually monitoring a receive user channel of said trunk at said network node to detect said bit pattern while said trunk is out of service, and means for identifying the presence of a loopback when said bit pattern is persistently detected on said receive user channel.
In one embodiment, a detector is provided to detect the out-of-service trunk.